1. Field of the Invention
The present invention relates to an electrochromic element and an optical filter, and particularly relates to an organic electrochromic element provided with a mechanism for controlling an optical path length, a method for driving the same, and an optical filter using the same.
2. Description of the Related Art
An electrochromic (EC) phenomenon is a phenomenon that an optical absorption region of a material changes as induced by a reversible electrochemical reaction (oxidation reaction or reduction reaction) which occurs upon application of a voltage, leading to coloration or decoloration of the material. An electrochemical coloring/decoloring element using such an electrochromic phenomenon is called an electrochromic element, and expected to be applied as a light modulating element for changing a light transmittance.
As an EC material, there are known one using a metal oxide such as WO3 and one using a conductive polymer. In Japanese Patent Application Laid-Open No. 51-146253, an EC element using an organic low-molecular material such as viologen is described. Among such elements, an organic EC element which colors/decolors a low-molecular organic material in a solution state is known to have an advantage of obtaining a sufficient contrast ratio in a colored state while obtaining a high transmittance in a decolored state.
The low-molecular organic EC material in the solution form has absorption in a visible light region by coloration, and one material generally has one or two absorption peaks in the visible light region. For this reason, color control is performed by mixing a plurality of materials having different absorption.
When the organic EC element is used as the light modulating element, a color scale (gradation) needs to be controlled. The gradation is set based upon an electrochemical reaction amount of the organic EC material, and the reaction amount is generally adjusted by electric control (applied voltage).
Further, in Chem. Mater., 1992, 4, pp. 1106-1113, a material which changes an absorption spectrum in the process of coloration by an oxidation reaction is reported.
The electrochromic element described in Japanese Patent Application Laid-Open No. 51-146253 is an element which controls a transmittance of an EC material by application of a voltage. It is described that controlling a voltage to be applied to the element leads to control of the electrochemical reaction of the EC material and control of its transmittance.
However, there are cases where just controlling the electrochemical reaction cannot lead to sufficient control of a light transmittance of the element.
The present invention was made in view of such a background, and provides an electrochromic element capable of controlling a light transmittance by changing a distance between electrodes of the electrochromic element, a method for driving the electrochromic element, and an optical filter.